Flat Panel Displays such as Liquid Crystal Displays typically use sheet polarizers. Most of the sheet polarizers in a commercial scale production are made by a unidirectional alignment of dichroic crystals in a sub-micron size. The crystals are introduced into a polymer material such as Polyvinyl Alcohol (PVA) followed by stretching to introduce an uni-axial alignment of the dichroic crystals. An “O”-type sheet polarizer absorbs the extraordinary ray vibrating in the direction of the stretch while transmitting the ordinary ray. The other type of sheet polarizer, “E”, transmits the extraordinary ray and absorbs the ordinary ray. Most of the sheet polarizers currently in a commercial use are “O” type. Note that there are many other types of polarizers, such as wire grid polarizers described in U.S. Pat. No. 6,122,103 commercially available from Moxtek Inc. of Orem, Utah, or prismatic polarizers as disclosed in U.S. Pat. No. 2,403,731, however, in this patent application, “polarizer” shall be understood as sheet polarizers only, such as high efficiency polarizer NPF-G1220DU available from Nitto Denko America, Inc. of San Jose, Calif. An ideal sheet polarizer would absorb 100% of light polarized in the absorbing axis and transmit 100% of light polarized in the transmission axis. However, a practical sheet polarizer generally transmits less than 50% of incoming un-polarized light. Usually, a sheet polarizer comprises an absorbing layer and two protective layers, and sometimes also includes compensators. For this reason, a polarizer will also be referred to as a polarizer package in this application.
In a typical transmissive LCD such as Twisted Nematic (TN) LCD, a liquid crystal cell is positioned between two crossed polarizer packages with their absorbing axes (or equally, transmission axes) orthogonal to each other. With continuous development of LCDs modes and their compensators, the apparent quality of LCDs can be limited by the performance of the polarizer package. N particular, the apparent performance of an LCD is much affected by the amount of light leakage traveling through the two crossed polarizer packages when the liquid crystal is not placed between the two crossed polarizers. It is desirable to have as little light leakage as possible. Actually, very little light can go through two crossed polarizer packages provided the light is impinged in the direction normal to the plane of the sheet polarizers. As is well known in the art, however, when the light propagation direction deviates from the normal, there occurs a significant amount of light leakage with the maximum leakage occurring at a large polar angle and 45 degrees of azimuthal angle relative to the transmission axis of the polarizer. In order to prevent this, the combination of two types of polarizers “O” type and “E” type has been suggested by Lazarev et al. (“Low-leakage off-angle in E-polarizers”, Journal of the SID, pp.101-105 (2001)). Theoretically, a matched combination of the two transmits little light in all directions. Yet, actual materialization of the “E”-type sheet polarizer is in its beginning state and its performance, by no means, is satisfactory. Thus, other methods of prevention of the leakage are desirable for practical applications. In what follows, a polarizer may be either an “O” or “E” type, although the specific examples provided actually are “O” type sheet polarizers.
Another approach to reduce light leakage through two crossed polarizer packages is to insert a compensator between the two polarizer packages. The compensators used for polarizers are nominally a combination of an A-plate and a C-plate, or two biaxial plates, as suggested by Chen et al. (“Optimum film compensation modes for TN and VA LCDs”, SID 98 Digest, pp.315-318 (1998)) and lshinabe et al. (“Novel wide viewing angle polarizer with high achromaticity”, SID 2000 Digest, pp.1094-1097 (2000); and “A wide viewing angle polarizer and a quarter-wave plate with a wide wavelength range for extremely high quality LCDs”, Asia Display/IDW 2001, pp.485-488 (2001)), respectively. Thus a wide viewing angle polarizer is realized. A wide viewing angle polarizer shall be understood as a polarizer package comprising an integrated combination of an absorbing layer (which polarizes light) and its compensators, which enables a transmissive optical device comprising two crossed polarizer packages with little light leakage from wide viewing angles. As discussed above, this transmissive optical device formed by two crossed polarizer packages is critical to the quality of an LCD.
By a careful examination of the designs of the transmissive optical device formed by wide viewing angle polarizers in the prior art, the inventors have found that in most cases, the prior art designs for the crossed polarizer packages cannot be separated into two identical parts containing the first polarizer package and the second crossed polarizer package. This is explained in FIG. 2D. A prior art transmissive optical device 29 is formed by two crossed polarizer packages 29A and 29B. The polarizer package 29A is constructed by an integrated combination of an absorbing layer 25A and a compensator 23A, while the polarizer package 29B is constructed by an integrated combination of an absorbing layer 25B and a compensator 23B. The arrows in the plane of absorbing layers 25A, 25B indicate their transmission axes and are orthogonal to each other. The compensators 23A and 23B are intended to prevent light 7 from leaking through the optical device 29, however, according to the prior art, the compensators 23A, 23B are different. Consequently, the resulting polarizer packages 29A, 29B, which are integrated combinations of an absorbing layer and a compensator, are not identical. This characteristic means that the package of the first polarizer is different from that of the second polarizer due to the use of two different compensators, and this optical device 29 will be referred as a non-identical compensators design device in this application.
As a result of this non-identical compensators design, the prior art requires a strict alignment for the compensator relative to the transmission axis of the absorbing layer, and therefore complex and precise manufacturing processes are employed. Such designs also require manufacturing two different compensators, and therefore an extra procedure in a production. Furthermore, a polarizer package design using non-identical compensators, such as suggested by Chen et al., can be used with a transmissive optical device, but not with a reflective optical device. Alternately, according to Ishinabe et al. (“A wide viewing angle polarizer and a quarter-wave plate with a wide wavelength range for extremely high quality LCDs”, Asia Display/IDW 2001, pp.485-488 (2001)), two different biaxial plates can be used effectively as compensators in a polarizer package for the front and the rear polarizers, respectively. Although two identical biaxial plates can be utilized in conjunction with the front and rear polarizers, the performance of crossed polarizer packages degrades, as the referenced publication states.
Yoshimi et al. disclosed a polarizer with an attached compensator in Japanese Patent 09325216A. The compensator can be put on at least one side of the absorbing layer. The film is of biaxial nature with an in-plane retardation 50 to 200 nm. The in-plane slow axis lies either parallel or perpendicular to the transmission axis of the polarizer. However, the principal function of the compensator in their invention is not to compensate the crossed polarizers, but rather to compensate for the light in an oblique direction through the liquid crystal cell so as to prevent color shift and contrast inversion. Therefore, the transmissive optical device formed by two such crossed polarizer packages alone may not prevent light leakage at a wide viewing angle.
Therefore it is desirable to provide enhanced polarizer packages which have reduced sensitivity to an alignment of its compensator relative to its transmission axis, and which can therefore be fabricated with a simpler and less demanding method.
It is also desirable to provide a polarizer package, which provides enhanced polarization performances by using substantially identical polarizer packages for both the front and rear polarizers to form a transmissive optical device. “Substantially the same polarizer package” means a polarizer package manufactured by the same method.
Furthermore, it is desirable to provide a polarizer package, which provides enhanced polarization performances not only effective in a transmissive optical device, but also in a reflective optical device comprising such a polarizer package, a quarter wave plate, and a reflective layer.
These high performance polarizer packages can be employed to improve viewing characteristics of a liquid crystal display. Also applications can be made, in combination with a wide viewing quarter wave plate, as an anti-reflection films for an emissive display, such as an Organic Light Emitting Diode display (OLED).